Buffer systems are widely used in biology and biochemistry to maintain the pH of solutions. While the pH of a buffered solution resists changes upon the addition of small amounts of an acid or base to the solution, the pH of conventional buffers displays marked temperature dependence. The basis for the observed temperature dependence may be attributed to temperature affecting the ionization of weak acids and bases commonly used in buffer systems, as well as the ionization of water, and/or to the selective precipitation of buffer ions from solution.
Cryogenic temperatures are often required for numerous biochemical and biophysical studies such as the use of electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and Mössbauer and X-ray absorption spectroscopy (XAS). In addition, X-ray diffraction data are collected at liquid nitrogen temperatures for protein structure determination. Aqueous buffer solutions are invariably used for these measurements, and low temperatures can change the buffer pH, which in turn, can modify the system inadvertently.
The cryopreservation of biological tissues, cells, and specimens is compromised by thermal fluctuations around the glassing temperature of water (−130° C.; 143 K). Biological and chemical activity can persist as long as water activity exists; however, all activity ceases below 143 K. Even the most temperature-sensitive cells are estimated to survive for hundreds of years when stored at temperatures below 143 K. However, the longevity of cells is reduced to months when stored above this temperature. Since the pH of cryogenic preservative solutions is determined at temperatures far above the glassing temperature of water, significant pH variations are expected as the solutions are subjected to the cryogenic process. Moreover, pH variations will persist whenever variations in temperature arise during long-term storage of cryogenic samples. Temperature dependent pH variation in cryopreservative solutions likely plays a contributory role in the integrity of cryogenic samples.
The stability of pharmaceutical compounds is also affected by temperature dependent pH changes. While solid and lyophilized powder formulations are relatively stable for prolonged periods of time, reconstituted or ready-to-use solutions of pharmaceutical compounds are particularly susceptible to inactivation owing to chemical breakdown over time. Low temperature storage of pharmaceutical solutions under freezing conditions (for example, −20° C. (253 K)) slows the temperature dependent chemical degradation of aqueous formulations. Nevertheless, many pH-sensitive pharmaceutical compounds are adversely affected when solutions containing these ingredients are subjected to repeated cycles of freeze-thaw treatment over time.
The biopharmaceutical industry expends considerable time and resources to develop formulations of pharmaceutical compounds that maintain their activity for prolonged periods of time without low temperature storage. Ready-to-use solution formulations are often preferred for pharmaceutical compounds that are extremely toxic, such as chemotherapeutic agents. The traditional approach to this problem focuses on the development of a ready-to-use solution that circumvents the instability issues peculiar to a chosen pharmaceutical compound for storage at refrigerant temperatures (for example, 4-8° C.). Storage stable, ready-to-use solutions have been achieved for certain pH-sensitive antitumor compounds, such as cisplatin and doxorubicin (Alam et al. 1990; Gatti et al. 2000). No temperature resistant pH buffer systems have been developed that may obviate the need to develop tailored ready-to-use formulations for specific pH-sensitive pharmaceutical compounds.
While it is widely accepted that the pH of buffer solutions changes with temperature, quantitative determination of the pH changes at cryogenic temperatures has not been achieved due to the absence of a suitable probe. Different methods have been used to monitor pH variation as a function of temperature. Measurement of the electromotive force has been used to determine variation of pKa values of phosphate ion in an ethanol/water mixture at temperatures down to −10° C. (Bates et al. 1980). This method was also used to determine temperature dependence of pKa of MOPS in a glycerol/water mixture for the temperature range 20 to 25° C. (Roy et al. 1985). Several pH-sensitive dyes have been used to probe the protonic activity of supercooled solutions of water-organic solvent mixtures (Hoa et al. 1973). Similar approaches were used to obtain a rough estimation of the pH changes of selected aqueous buffer solutions upon freezing (Williams-Smith et al. 1977; Orii et al. 1977).
Commonly used pH electrodes have limited utility where pH measurements are performed over a broad temperature range, including extremely low temperatures. A preferred method of measuring pH includes monitoring the optical spectrum of solutions. However, some optical pH-indicators display a temperature dependence that hampers their utility in photometric pH measurements. For example, the spectral properties of the pH-indicators phenol red, bromocresol purple, and bromothymol blue change as a function of both temperature and pH (Hafeman et al. 1993). Buffer/pH-indicator compositions are described for these compounds that display temperature-independent spectral properties for the included pH indicator (id.). It is not clear whether these buffer/pH-indicator compositions can be used to colorimetrically monitor solution pH at different temperatures, owing to the presence of the buffer in the composition. Thus, pH-sensitive dyes whose spectral properties reflect changes in only the pH of the solution are desired.
The temperature effect on pH was originally recognized for low temperature storage of solutions containing proteins. Over seventy years ago, Finn reported that the denaturation of proteins contained in muscle juice was attributed to the variation in hydrogen ion and salt concentrations upon freezing (Finn 1932). Over forty years ago, Chilson and coworkers observed loss of enzymatic activity of aldolase and dehydrogenase in sodium and potassium phosphate buffer solutions upon freezing and thawing, which they attributed to the inactivation of the enzymes by pH changes (Chilson et al. 1965).
More recently, it was recognized that pH-dependent denaturation of proteins can occur by as little as a 1-2 unit variation in pH. For example, thrombin-activated porcine factor VIII is a stable, active, heterotrimer at pH 6.0 at 4° C. or 20° C. However, this protein undergoes a sharp decline in coagulation activity between pH 7 and 8. Furthermore, the coagulation activity of the protein cannot be restored by readjusting the pH to 6.0. The loss of activity correlates with dissociation and precipitation of individual subunits of the trimeric protein. Lollar and Parker (1990) proposed that the denaturation of the protein by pH-dependent subunit dissociation may be a major mechanism for the inactivation of the protein under physiologic conditions.
Thus, subtle variation in solution pH can dramatically alter protein structure and function. The pH of storage solutions for proteins or other pH-sensitive macromolecules can vary dramatically as a function of temperature, thereby affecting their stability.
The problem of buffered solutions displaying marked pH variation as a function of temperature has been long recognized. Yet, surprisingly, there are no published attempts to systematically develop buffer systems that resist temperature dependent variation of pH. Research applications, cryopreservation methodology, and pH-sensitive pharmaceutical compound formulations would certainly benefit from the availability of such buffer systems. Thus, there has been a long felt need for buffer systems that display temperature resistant pH characteristics.